Method and apparatus for phrenic stimulation detection

ABSTRACT

Approaches for characterizing a phrenic stimulation threshold, a cardiac capture threshold, a maximum device parameter, and a minimum device parameter are described. A plurality of cardiac pacing pulses can be delivered by using a cardiac pacing device, a pacing parameter of the plurality of cardiac pacing pulses being changed between delivery of at least some of the pulses. One or more sensor signals can be evaluated to detect stimulation of the phrenic nerve by one or more of the plurality of cardiac pacing pluses. The evaluation of the one or more sensor signals and the pacing parameter can be compared to determine if a phrenic stimulation threshold is at least one of higher than a maximum device parameter and lower than a minimum device parameter.

This application is a continuation of U.S. patent application Ser. No.14/665,690, filed Mar. 23, 2015, which is a continuation of U.S. patentapplication Ser. No. 14/157,571, filed Jan. 17, 2014, now issued as U.S.Pat. No. 8,996,112, which is a continuation of U.S. patent applicationSer. No. 12/368,828, filed Feb. 10, 2009, now U.S. Pat. No. 8,649,866,which claims the benefit of Provisional Patent Application Ser. No.61/065,743 filed on Feb. 14, 2008, to which priority is claimed pursuantto 35 U.S.C. §119(e), all of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to cardiac devices and methods,and, more particularly, to characterization of capture and phrenicstimulation thresholds and device parameters.

BACKGROUND OF THE INVENTION

When functioning normally, the heart produces rhythmic contractions andis capable of pumping blood throughout the body. The heart hasspecialized conduction pathways in both the atria and the ventriclesthat enable excitation impulses (i.e. depolarizations) initiated fromthe sino-atrial (SA) node to be rapidly conducted throughout themyocardium. These specialized conduction pathways conduct thedepolarizations from the SA node to the atrial myocardium, to theatrio-ventricular node, and to the ventricular myocardium to produce acoordinated contraction of both atria and both ventricles.

The conduction pathways synchronize the contractions of the musclefibers of each chamber as well as the contraction of each atrium orventricle with the opposite atrium or ventricle. Without thesynchronization afforded by the normally functioning specializedconduction pathways, the heart's pumping efficiency is greatlydiminished. Patients who exhibit pathology of these conduction pathwayscan suffer compromised cardiac output.

Cardiac rhythm management (CRM) devices have been developed that providepacing stimulation to one or more heart chambers in an attempt toimprove the rhythm and coordination of atrial and/or ventricularcontractions. CRM devices typically include circuitry to sense signalsfrom the heart and a pulse generator for providing electricalstimulation to the heart. Leads extending into the patient's heartchamber and/or into veins of the heart are coupled to electrodes thatsense the heart's electrical signals and deliver stimulation to theheart in accordance with various therapies for treating cardiacarrhythmias and dyssynchrony.

Pacemakers are CRM devices that deliver a series of low energy pacepulses timed to assist the heart in producing a contractile rhythm thatmaintains cardiac pumping efficiency. Pace pulses may be intermittent orcontinuous, depending on the needs of the patient. There exist a numberof categories of pacemaker devices, with various modes for sensing andpacing one or more heart chambers.

A pace pulse must exceed a minimum energy value, or capture threshold,to “capture” the heart tissue, generating an evoked response thatgenerates a propagating depolarization wave that results in acontraction of the heart chamber. If a pace pulse energy is too low, thepace pulses may not reliably produce a contractile response in the heartchamber and may result in ineffective pacing that does not improvecardiac function or cardiac output.

Pacing pulses can unintentionally stimulate nerves or muscles, even ifthe pulse energy is not sufficient to capture cardiac tissue. Forexample, a delivered pacing pulse may stimulate a patient's phrenicnerve, which runs proximate the heart and innervates the diaphragm.

The present invention provides methods and systems using phrenicstimulation algorithms and provides various advantages over the priorart.

SUMMARY OF THE INVENTION

The present invention involves approaches for using phrenic stimulationalgorithms for characterization of capture and phrenic stimulationthresholds and device parameters. One embodiment of the invention isdirected to a method comprising delivering a plurality of cardiac pacingpulses using a cardiac pacing device, a pacing parameter of theplurality of cardiac pacing pulses being changed between delivery of thepulses. The parameter can be a pacing pulse amplitude or width, forexample. The method embodiment further includes evaluating one or moresensor signals to detect activation of the phrenic nerve by one or moreof the plurality of cardiac pacing pulses and comparing the evaluationof the one or more sensor signals and the pacing parameter to determineif a phrenic nerve activation threshold is at least one of higher than amaximum device parameter and lower than a minimum device parameter. Thecardiac capture threshold can be a detected minimum pacing pulseamplitude that causes depolarization in targeted cardiac tissue. Themaximum device parameter can be a maximum width of a pacing pulse thatthe cardiac pacing device is programmed to deliver. The minimum deviceparameter can be a minimum width of a pacing pulse that the cardiacpacing device is programmed to deliver.

Another embodiment is directed to a cardiac rhythm management system,the system comprising an implantable cardiac pacing device having aplurality of electrodes. The implantable cardiac pacing device caninclude circuitry configured to output a plurality of cardiac pacingpulses through the electrodes and modify one or more pacing parametersof the plurality of cardiac pacing pulses, one or more sensorsconfigured to sense activation of the phrenic nerve by one or more ofthe plurality of pacing pulses and provide one or more signals based onthe sensed phrenic nerve activation, a controller configured to executeprogram instructions stored in memory to cause the system to compare theone or more signals and the one or more pacing parameters to determineif a phrenic nerve activation threshold is at least one of higher than amaximum cardiac pacing device parameter and lower than a minimum cardiacpacing device parameter, and store information based on thedetermination.

Another embodiment is directed to a cardiac rhythm management system,the system comprising an implantable cardiac pacing device having aplurality of electrodes. The implantable cardiac pacing device caninclude circuitry configured to output a plurality of cardiac pacingpulses through the plurality of electrodes and modify one or more pacingparameters of the plurality of cardiac pacing pulses, one or moresensors configured to sense activation of the phrenic nerve by one ormore of the plurality of pacing pulses and provide one or more signalsbased on the sensed phrenic nerve activation, means for comparing theevaluation of the one or more sensor signals and the one or more pacingparameters to determine if a phrenic nerve activation threshold is atleast one of higher than a maximum programmed device parameter and lowerthan a minimum programmed device parameter.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a method of characterizing deviceparameter limits, capture thresholds, and phrenic stimulation thresholdsin accordance with embodiments of the invention;

FIG. 2 is a block diagram of system circuitry in accordance withembodiments of the invention;

FIG. 3 is a diagram illustrating a patient-external device that providesa user interface allowing a human analyst to interact with informationand program an implantable medical device in accordance with embodimentsof the invention;

FIG. 4 is a therapy device incorporating circuitry capable ofimplementing electrode combination selection techniques in accordancewith embodiments of the invention;

FIG. 5 is a flowchart illustrating a method of estimating thresholds inaccordance with embodiments of the invention;

FIG. 6 is a graph illustrating various aspects of strength-durationpacing pulse parameter and device limit curves in accordance withembodiments of the invention;

FIG. 7 is a flowchart illustrating a method characterizing deviceparameter limits, capture thresholds, and phrenic stimulation thresholdsusing a step-up scanning technique in accordance with embodiments of theinvention;

FIG. 8 is a flowchart illustrating a method of characterizing deviceparameter limits, capture thresholds, and phrenic stimulation thresholdsusing a step-down scanning technique in accordance with embodiments ofthe invention; and

FIG. 9 is a flowchart illustrating a method of characterizing a capturethreshold and phrenic stimulation threshold using a step-down scanningtechnique in accordance with embodiments of the invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings forming a part hereof, and inwhich are shown by way of illustration, various embodiments by which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

Systems, devices, or methods according to the present invention mayinclude one or more of the features, structures, methods, orcombinations thereof described herein. For example, a device or systemmay be implemented to include one or more of the advantageous featuresand/or processes described below. It is intended that such a device orsystem need not include all of the features described herein, but maybeimplemented to include selected features that provide for usefulstructures and/or functionality. Such a device or system may beimplemented to provide a variety of therapeutic or diagnostic functions.

A wide variety of implantable cardiac monitoring and/or stimulationdevices may be configured to implement phrenic stimulation algorithms ofthe present invention. A non-limiting, representative list of suchdevices includes cardiac monitors, pacemakers, cardiovertors,defibrillators, resynchronizers, and other cardiac monitoring andtherapy delivery devices. These devices may be configured with a varietyof electrode arrangements, including transveneous, endocardial, andepicardial electrodes (i.e., intrathoracic electrodes), and/orsubcutaneous, non-intrathoracic electrodes, including can, header, andindifferent electrodes, and subcutaneous array(s) or lead electrodes(i.e., non-intrathoracic electrodes).

Bi-ventricular pacing provides therapy options for patients sufferingfrom heart failure. However, new challenges have been presented byplacement of the left-ventricular lead via the coronary sinus inbi-ventricular pacing systems. Due to the proximity of the coronaryveins to the phrenic nerve, left ventricular pacing may result inphrenic nerve stimulation. The phrenic nerve innervates the diaphragm,so stimulation of the phrenic nerve can cause a patient to experience ahiccup. Electrical stimulation of the phrenic nerve can be uncomfortablefor the patient, and can interfere with breathing. Therefore, phrenicstimulation from cardiac pacing may cause the patient to exhibituncomfortable breathing patterns timed with the left-ventricular pace.

A phrenic stimulation threshold, above which the phrenic nerve will bestimulated by a pacing pulse, can be determined. One method fordetermining a phrenic stimulation threshold includes sensing for phrenicnerve activation and/or diaphragmic movement timed with the delivery ofpacing pulses. If no phrenic stimulation is sensed using the level ofelectrical energy delivered, the energy level can be iterativelyincreased for subsequent trials of delivering electrical energy andmonitoring for phrenic stimulation until phrenic stimulation is sensed.The electrical energy level at which phrenic stimulation is detected canbe the phrenic stimulation threshold. In some embodiments, the level ofelectrical energy may be decreased or otherwise adjusted until phrenicstimulation is not detected. The energy delivered during such a scancould also be used to simultaneously perform other tests, such assearching for a capture threshold.

Methods for evaluating phrenic stimulation that maybe incorporated inembodiments of the invention are disclosed in U.S. Pat. No. 6,772,008;and Patent Application Publication No. 20060241711, filed Apr. 26, 2005,each of which are herein incorporated by reference in their respectiveentireties.

Programming a pacing device to avoid undesirable stimulation, such asphrenic stimulation, is not one dimensional, as many other factors canbe important in setting appropriate pacing parameters. For example, apace pulse must exceed a minimum energy value, or capture threshold, toproduce an intended contraction of cardiac tissue. It is desirable for apace pulse to have sufficient energy to stimulate capture of the heartwithout expending energy significantly in excess of the capturethreshold. Thus, accurate determination of the capture thresholdprovides efficient pace energy management. If the pace pulse energy istoo low, the pace pulses may not reliably produce a contractile responsein the heart and may result in ineffective pacing.

A capture threshold can be determined using, among other methods, astep-down technique where a capture threshold is identified when loss ofcapture is detected after successive pacing cycles. A step-up techniquecan also be used, whereby a capture threshold is identified when captureis detected after successive pacing cycles without capture. Capture canbe detected using characteristics of an electrocardiogram indicating anintended cardiac response (e.g., a QRS complex).

Capture detection allows the cardiac rhythm management system to adjustthe energy level of pace pulses to correspond to the optimum energyexpenditure that reliably produces a contraction. Further, capturedetection allows the cardiac rhythm management system to initiate aback-up pulse at a higher energy level whenever a pace pulse does notproduce a contraction. For example, embodiments of the invention cancharacterize capture and phrenic stimulation thresholds and deliver aback-up pulse with a higher energy level than a standard pacing energylevel set at or above the capture threshold, the back-up pulse energylevel below the phrenic stimulation threshold.

Pacing devices can have pacing parameter limits, such as programmedlimits and limits on device capability. For example, an implantablepacing device may be programmed to only deliver pacing pulses notexceeding a specified amplitude. Other programmed limits can includeminimum pulse amplitude, minimum pulse duration, maximum pulse duration,minimum pulse frequency, maximum pulse frequency, minimum pulse current,and/or maximum pulse current, among other parameters. Devices may beprogrammed with these and other parameter limits for several reasons.For example, a device programmed to automatically select a pacing pulseamplitude may have a programmed limit on the amplitude to prevent thedevice from selecting a pulse amplitude that could be dangerous to thepatient (such as in an auto-capture mode). Additionally, a device may beprogrammed with parameter limits to avoid operating conditions that canharm the device.

Upon implantation of a pacing device, a human analyst, such as a doctor,may establish initial capture and phrenic nerve stimulation thresholds.Based on those initial threshold determinations, the analyst may programthe device with parameter range limits within which automated devicefeatures can operate. In the case of pulse amplitude, a doctor may set avoltage range having a minimum above the capture threshold and a maximumbelow the phrenic stimulation threshold within which an autocapturefeature can operate. The maximums and minimums may each have a safetymargin such that the range between the parameter minimum/maximum is lessthan the range between the capture and phrenic stimulation thresholds.The safety margin provides some protection from an autocapture programpacing below the capture threshold or above the phrenic stimulationthreshold if either of these thresholds were to change over time.

Devices can also have capability limits associated with the hardware ofthe device. For example, even if no programming limits are placed on theoperation of a pacing device, the device may still have limits regardingminimum pulse amplitude, maximum pulse amplitude, minimum pulseduration, maximum pulse duration, minimum pulse frequency, maximum pulsefrequency, minimum pulse current, and maximum pulse current, among otherparameters. Device capability limits can be related to the performancelimits of the components, such as capacitors and battery, used toconstruct the device.

In multi-electrode pacing systems, multiple pacing electrodes may bedisposed in a single heart chamber, in multiple heart chambers, and/orelsewhere in a patient's body. Electrodes used for delivery of pacingpulses may include one or more cathode electrodes and one or more anodeelectrodes. Pacing pulses are delivered via the cathode/anode electrodecombinations, where the term “electrode combination” denotes that atleast one cathode electrode and at least one anode electrode are used.An electrode combination may involve more than two electrodes, such aswhen multiple electrodes that are electrically connected are used as theanode and/or multiple electrodes that are electrically connected areused as the cathode. Typically, pacing energy is delivered to the hearttissue via the cathode electrode(s) at one or more pacing sites, with areturn path provided via the anode electrode(s). If cardiac captureoccurs, the energy injected at the cathode electrode site creates apropagating wavefront of depolarization which may combine with otherdepolarization wavefronts to trigger a contraction of the cardiacmuscle. The cathode and anode electrode combination that delivers thepacing energy defines the pacing vector used for pacing.

Pacing pulses maybe applied through multiple electrodes (i.e., pacingvectors defined by various electrode combinations) in a single cardiacchamber in a timed sequence during the cardiac cycle to improvecontractility and enhance the pumping action of the heart chamber. It isdesirable for each pacing pulse delivered via the multiple electrodecombinations to capture the cardiac tissue proximate the cathodeelectrode. Capture of cardiac tissue depends upon, among other things,the vector used to deliver the pulse and various pulse parameters, suchas the amplitude and duration of the pulse.

Stimulation characteristics of a pacing therapy are dependent on manyfactors, including the distance between the electrodes, proximity totargeted tissue, proximity to non-targeted tissue susceptible tounintended stimulation, type of tissue contacting and between theelectrodes, impedance between the electrodes, resistance between theelectrodes, and electrode type, among other factors. Such factors caninfluence the cardiac capture and phrenic stimulation thresholds.Stimulation characteristics can vary with physiologic changes, electrodemigration, physical activity level, body fluid chemistry, hydration, anddisease state, among other factors. Therefore, the stimulationcharacteristics for each electrode combination are unique and can changeover time. As such, it can be useful to periodically determine thestimulation characteristics (e.g., cardiac capture and phrenicstimulation thresholds) for each electrode combination for optimumpacing (e.g., pacing at or just above the cardiac capture threshold andnot causing undesirable stimulation).

It can be useful to consider device parameter limitations in relation tovarious thresholds (e.g., phrenic stimulation, cardiac capture) whenprogramming, reprogramming, and/or operating a device. For example, ifit is known that the phrenic stimulation threshold is greater than adevice pulse amplitude limit, then a device may not need to considerphrenic stimulation when performing a scan to update a cardiac capturethreshold. Additionally, a step-up amplitude scan of an auto-captureprocedure may use larger parameter increments to facilitate fasterdetermination of a cardiac capture threshold if it is known that thephrenic stimulation threshold is higher than the programmed devicelimits. In a step-down scan mode, a device may alert a physician and/orperform a reconfiguration if it is determined that the device minimumparameter limits are above the phrenic stimulation threshold (e.g., thedevice cannot deliver a pacing pulse having a pulse width that does notstimulate the phrenic nerve).

Devices of the present invention may facilitate characterization ofpacing configurations using various phrenic stimulation algorithms. Adevice may determine the relationship between device parameter limits,capture threshold, and/or phrenic stimulation threshold, among otherthings. Embodiments may notify a doctor and/or take some other action ifchanges in one or more of the thresholds might change the relationshipbetween the programmed parameter limits and one or more of thethresholds. For example, a doctor might be notified and/or a device maybe automatically reprogrammed (by itself or another system) if a phrenicstimulation threshold changes such that the threshold is lower than aprogrammed pacing parameter limit, where before the phrenic stimulationthreshold was higher than the maximum parameter limit of the programmedrange.

The flowchart of FIG. 1 illustrates a process for using a phrenicstimulation algorithm. The process includes delivering 110 cardiacpacing pulses while iteratively changing a parameter of the pacingpulses. The parameter iteratively changed could be one or more of pulseamplitude, width, frequency, and current, among other parameters. Theparameter change could be an increase or decrease. In this way, a scancan be performed for each of the one or more parameters to investigatethe physiological response (e.g., capture, phrenic stimulation) acrossat least a portion of the available parameter spectrum.

Sensor signals are evaluated 120 to detect phrenic stimulation by thedelivered 110 cardiac pacing pulses. Phrenic stimulation can be detectedby the methods disclosed herein. In one embodiment, phrenic stimulationcould be detected by an accelerometer signal indicating thoracicmovement (e.g., an induced hiccup) shortly after the delivery 110 of apacing pulse. Phrenic stimulation can also be detected by the detectionof a short-duration deviation in the amplitude of a transthoracicimpedance signal.

Information regarding the evaluation 120 to detect phrenic stimulationand the delivery 110 of pacing pulses can be compared 130 to determine,among other things, if a phrenic stimulation threshold is higher than amaximum programmed device parameter, lower than a minimum programmeddevice parameter, and/or lower than cardiac capture threshold.

For example, if the evaluation 120 did not detect phrenic stimulationcorresponding to any of the delivered 110 pacing pulses, even though apacing pulse was delivered at the maximum programmed amplitude settingfor a pacing device, then it can be identified that the phrenicstimulation threshold is greater than the device's maximum programmedamplitude parameter. Additionally, it may be determined that the phrenicstimulation pulse width threshold is greater than the device's maximumprogrammed pulse width parameter. Similar relationships could also beidentified for the other parameters disclosed herein.

If the process of FIG. 1 is employed using a step-down iterativeparameter change approach, then it may be identified that the phrenicstimulation threshold is below a device's programmed minimum amplitudeand/or pulse width parameter. A step-down scan according to the processof FIG. 1 may also identify that a capture threshold is below a device'sminimum programmed amplitude and/or pulse width parameter, that aphrenic stimulation threshold is greater than a cardiac capturethreshold, and/or that either or both of the phrenic stimulation andcardiac capture thresholds are below a maximum programmed deviceparameter.

In some embodiments, a device may scan only within its programmedparameter range that a particular process, such as autocapture, isallowed to operate. Such embodiments minimize testing while ensuringthat a threshold has not drifted into the programmed range of theautomated process.

In some embodiments, a device may scan outside of its programmedparameter range within which a particular process is allowed to operate.Such embodiments allow a device and/or doctor to recognize whenthresholds have changed relative to programmed parameter limits and takeappropriate action. For example, if it is identified that a capturethreshold has decreased and drifted further from a minimum programmedparameter limit, then a device may be reprogrammed with a lower minimumparameter limit, which can conserve battery life. If a phrenicstimulation threshold has decreased, then a device can be reprogrammedwith a lower maximum parameter limit to ensure a safety margin existsbetween the phrenic stimulation threshold and the maximum programmedparameter limit.

Based on the identified relationships between the phrenic stimulationthreshold, the capture threshold, and the various programmed pulseparameter limits, a doctor may be notified that reprogramming of thedevice is needed. For example, if it is determined that a capturethreshold has increased over time then embodiments of the invention canfacilitate notification to the doctor that the capture threshold hasincreased and is nearing a device's minimum programmed pulse amplitudeparameter (where a scan had been performed outside of the programmedparameter limits). In such case the doctor may reprogram the pulseparameter limits of the device by increasing the minimum programmedpulse parameter. A doctor may further increase the maximum programmedpulse parameter to allow automated functions of the device, such as anautocapture process, adequate variability in changing a parameter.Alternatively, a device may reprogram itself or be programmed by anothersystem to modify programmed parameter limits in response to identifiedchanging relationships between the phrenic stimulation threshold, thecapture threshold, and various programmed pulse parameter limits.

The various steps of FIG. 1, as well as the other steps disclosedherein, can be performed automatically, such that no direct humanassistance (e.g., physician and/or patient) is needed to initiate orperform the various discrete steps. Alternatively, the various steps ofthis disclosure can be performed semi-automatically requiring someamount of human interaction to initiate or conduct one or more steps.

FIG. 2 is a block diagram of a CRM device 200 that may incorporatecircuitry employing phrenic stimulation algorithms in accordance withembodiments of the present invention. The CRM device 200 includes pacingtherapy circuitry 230 that delivers pacing pulses to a heart. The CRMdevice 200 may optionally include defibrillation/cardioversion circuitry235 configured to deliver high energy defibrillation or cardioversionstimulation to the heart for terminating dangerous tachyarrhythmias.

The pacing pulses are delivered via multiple cardiac electrodes 205(electrode combinations), which can be disposed at multiple locationswithin a heart, among other locations. Two or more electrodes may bedisposed within a single heart chamber. The electrodes 205 are coupledto switch matrix 225 circuitry used to selectively couple electrodes 205of various pacing configurations to signal processor 201 and/or othercomponents of the CRM device 200.

The CRM device also includes a phrenic stimulation sensor 210. Thephrenic stimulation sensor 210 can output a signal and/or otherinformation to signal processor 201 and control processor 240. Phrenicstimulation sensor 210 may include an accelerometer, electrical signalsensors (e.g., EMG, impedance), pressure sensor, acoustic sensors,and/or any other sensor that can participate in the detection of phrenicstimulation. Phrenic stimulation sensor 210 may be implemented using adiscrete sensor or via software executed by a processor (e.g., controlprocessor 240) of the FRM device.

The control processor 240 can use information received from the signalprocessor 201, the phrenic stimulation sensor 210, memory 245, and othercomponents to implement phrenic stimulation algorithms, as disclosedherein.

For example, the pacing therapy circuitry 230 can provide informationregarding when a pacing pulse was delivered and the parameters of thepacing pulse, the phrenic stimulation sensor 210 can provide informationregarding sensed phrenic stimulation, and signal processor can provideinformation regarding capture of the heart. This information can be usedto determine, among other things, if a phrenic stimulation threshold ishigher than a maximum device parameter, lower than a minimum deviceparameter, and/or lower than cardiac capture threshold, among otherthings.

Amplitude, peak timing, and/or correlation of delivered pulses tophrenic stimulation (beat-to-beat and/or over time) can be used with aphrenic stimulation signal in either the time or frequency domain todetermine whether one or more pacing pulses caused phrenic stimulation.

A CRM device 200 typically includes a battery power supply (not shown)and communications circuitry 250 for communicating with an externaldevice programmer 260 or other patient-external device. Information,such as data, parameter information, evaluations, comparisons, data,and/or program instructions, and the like, can be transferred betweenthe device programmer 260 and patient management server 270, CRM device200 and the device programmer 260, and/or between the CRM device 200 andthe patient management server 270 and/or other external system. In someembodiments, the processor 240, memory 245, and/or signal processor 201may be components of the device programmer 260, patient managementserver 270, and/or other patient external system.

The CRM device 200 also includes a memory 245 for storing executableprogram instructions and/or data, accessed by and through the controlprocessor 240. In various configurations, the memory 245 may be used tostore information related to thresholds, parameters, parameter limits,measured values, program instructions, and the like.

The circuitry represented in FIG. 2 can be used to perform the variousmethodologies and techniques discussed herein. Memory 245 can be acomputer readable medium encoded with a computer program, software,firmware, computer executable instructions, instructions capable ofbeing executed by a computer, etc. to be executed by circuitry, such ascontrol processor 240. For example, memory 245 can be a computerreadable medium storing a computer program, execution of the computerprogram by control processor 240 causing delivery of pacing pulsesdirected by the pacing therapy circuitry, reception of one or moresignals from phrenic stimulation sensors 210 and/or signal processor 201to identify, and establish relationships between, device parameterlimits, capture thresholds, and phrenic stimulation thresholds inaccordance with embodiments of the invention according to the variousmethods and techniques made known or referenced by the presentdisclosure. In similar ways, the other methods and techniques discussedherein can be performed using the circuitry represented in FIG. 2.

FIG. 3 illustrates a patient external device 300 that provides a userinterface configured to allow a human analyst, such as a physician orpatient, to interact with an implanted medical device. The patientexternal device 300 is described as a CRM programmer, although themethods of the invention are operable on other types of devices as well,such as portable telephonic devices, computers, PDA's, or patientinformation servers used in conjunction with a remote system, forexample. The programmer 300 includes a programming head 310 which isplaced over a patient's body near the implant site of an implanteddevice to establish a telemetry link between a CRM and the programmer300. The telemetry link allows the data collected by the implantabledevice to be downloaded to the programmer 300. The downloaded data isstored in the programmer memory 365. In some embodiments, acommunication link may be established between an implantable device andan external device via radio frequency, such that the implantable deviceand external device do not require relatively close proximity tofacilitate transfer of data, commands, instructions, and otherinformation.

The programmer 300 includes a graphics display screen 320, e.g., LCDdisplay screen, that is capable of displaying graphics, alphanumericsymbols, and/or other information. For example, the programmer 300 maygraphically display information regarding pacing parameters, devicelimits, sensed information, and thresholds downloaded from the CRM onthe screen 320. The display screen 320 may include touch-sensitivecapability so that the user can input information or commands bytouching the display screen 320 with a stylus 330 or the user's finger.Alternatively, or additionally, the user may input information orcommands via a keyboard 340 or mouse 350.

The programmer 300 includes a data processor 360 including softwareand/or hardware for performing the methods disclosed here, using programinstructions stored in the memory 365 of the programmer 300. In oneimplementation, sensed data is received from a CRM via communicationscircuitry 366 of the programmer 300 and stored in memory 365. The dataprocessor 360 evaluates the sensed data, which can include informationrelated to pacing parameters, device limits, and thresholds. The dataprocessor 360 can also perform other method steps discussed herein,including evaluating signals, detecting phrenic stimulation, andcomparing pacing parameters, device limits, and thresholds, among otherthings. Pacing parameters, device limits, programmed parameter limits,and thresholds, as well as other information, may be presented to a uservia a display screen 320. A notification regarding device pacing pulseparameter limits, capture threshold, and phrenic stimulation thresholdcan be displayed using the display screen 320 for review by a humananalyst.

The therapy device 400 illustrated in FIG. 4 employs circuitry capableof implementing phrenic stimulation algorithm techniques describedherein. The therapy device 400 includes CRM circuitry enclosed within animplantable housing 401. The CRM circuitry is electrically coupled to anintracardiac lead system 410. Although an intracardiac lead system 410is illustrated in FIG. 4, various other types of lead/electrode systemsmay additionally or alternatively be deployed. For example, thelead/electrode system may comprise and epicardial lead/electrode systemincluding electrodes outside the heart and/or cardiac vasculature, suchas a heart sock, an epicardial patch, and/or a subcutaneous systemhaving electrodes implanted below the skin surface but outside theribcage.

Portions of the intracardiac lead system 410 are shown inserted into thepatient's heart. The lead system 410 includes cardiac pace/senseelectrodes 451-456 positioned in, on, or about one or more heartchambers for sensing electrical signals from the patient's heart and/ordelivering pacing pulses to the heart. The intracardiac sense/paceelectrodes, such as those illustrated in FIG. 4, may be used to senseand/or pace one or more chambers of the heart, including the leftventricle, the right ventricle, the left atrium and/or the right atrium.The CRM circuitry controls the delivery of electrical stimulation pulsesdelivered via the electrodes 451-456. The electrical stimulation pulsesmay be used to ensure that the heart beats at a hemodynamicallysufficient rate, may be used to improve the synchrony of the heartbeats, may be used to increase the strength of the heart beats, and/ormay be used for other therapeutic purposes to support cardiac functionconsistent with a prescribed therapy.

The lead system 410 may include defibrillation electrodes 441, 442 fordelivering defibrillation/cardioversion pulses to the heart.

The left ventricular lead 405 incorporates multiple electrodes 454 a-454d and 455 positioned at various locations within the coronary venoussystem proximate the left ventricle. Stimulating the ventricle atmultiple locations in the left ventricle or at a single selectedlocation may provide for increased cardiac output in patients sufferingfrom heart failure (HF), for example, and/or may provide for otherbenefits. Electrical stimulation pulses may be delivered via theselected electrodes according to a timing sequence and outputconfiguration that enhances cardiac function. Although FIG. 4illustrates multiple left ventricle electrodes, in other configurations,multiple electrodes may alternatively or additionally be provided in oneor more of the right atrium, left atrium, and right ventricle.

Portions of the housing 401 of the implantable device 400 may optionallyserve as one or more multiple can 481 or indifferent 482 electrodes. Thehousing 401 is illustrated as incorporating a header 489 that may beconfigured to facilitate removable attachment between one or more leadsand the housing 401. The housing 401 of the therapy device 400 mayinclude one or more can electrodes 481. The header 489 of the therapydevice 400 may include one or more indifferent electrodes 482. The can481 and/or indifferent 482 electrodes may be used to deliver pacingand/or defibrillation stimulation to the heart and/or for sensingelectrical cardiac signals of the heart.

Communications circuitry is disposed within the housing 401 forfacilitating communication between the CRM circuitry and apatient-external device, such as an external programmer or advancedpatient management (APM) system. The therapy device 400 may also includesensors and appropriate circuitry for sensing a patient's metabolic needand adjusting the pacing pulses delivered to the heart to accommodatethe patient's metabolic need.

In some implementations, an APM system may be used to perform some ofthe processes discussed here, including evaluating, estimating,comparing, detecting, selecting, and updating, among others. Methods,structures, and/or techniques described herein, may incorporate variousAPM related methodologies, including features described in one or moreof the following references: U.S. Pat. Nos. 6,221,011; 6,270,457;6,277,072; 6,280,380; 6,312,378; 6,336,903; 6,358,203; 6,368,284;6,398,728; and 6,440,066, which are hereby incorporated herein byreference in each of their respective entireties.

In certain embodiments, the therapy device 600 may include circuitry fordetecting and treating cardiac tachyarrhythmia via defibrillationtherapy and/or anti-tachyarrhythmia pacing (ATP). Configurationsproviding defibrillation capability may make use of defibrillation coils441, 442 for delivering high energy pulses to the heart to terminate ormitigate tachyarrhythmia.

CRM devices using multiple electrodes, such as illustrated herein, arecapable of delivering pacing pulses to multiple sites of the atriaand/or ventricles during a cardiac cycle. Certain patients may benefitfrom activation of parts of a heart chamber, such as a ventricle, atdifferent times in order to distribute the pumping load and/ordepolarization sequence to different areas of the ventricle. Amulti-electrode pacemaker has the capability of switching the output ofpacing pulses between selected electrode combinations within a heartchamber during different cardiac cycles.

Commonly owned U.S. Pat. No. 6,772,008, which is incorporated herein byreference, describes methods and systems that may be used in relation todetecting undesirable tissue stimulation. Muscle stimulation may bedetected, for example, through the use of an accelerometer and/or othercircuitry that senses accelerations indicating muscle movements thatcoincide with the output of the stimulation pulse.

Other methods of measuring tissue stimulation may involve, for example,the use of an electromyogram sensor (EMG), microphone, and/or othersensors. For example, stimulation of the laryngeal muscles may beautomatically detected using a microphone to detect the patient'sexpiration response to undesirable diaphragmic activation due toelectrical phrenic stimulation.

Undesirable nerve or muscle stimulation may be detected by sensing aparameter that is directly or indirectly responsive to the stimulation.Undesirable nerve stimulation, such as stimulation of the vagus orphrenic nerves, for example, may be directly sensed usingelectroneurogram (ENG) electrodes and circuitry to measure and/or recordnerve spikes and/or action potentials in a nerve. An ENG sensor maycomprise a neural cuff and/or other type or neural electrodes located onor near the nerve of interest. For example, systems and methods fordirect measurement of nerve activation signals are discussed in U.S.Pat. Nos. 4,573,481 and 5,658,318 which are incorporated herein byreference in their respective entireties. The ENG may comprise a helicalneural electrode that wraps around the nerve (e.g., phrenic nerve) andis electrically connected to circuitry configured to measure the nerveactivity. The neural electrodes and circuitry operate to detect anelectrical activation (action potential) of the nerve followingapplication of the electrical stimulation pulse.

Neural activation can be detected by sensing a surrogate parameter thatis indirectly responsive to nerve stimulation. Lung pressure, pleuralpressure, thoracic pressure, airway pressure, and thoracic impedance areexamples of parameters that change responsive to stimulation of thephrenic nerve. In some embodiments, a patient's airway pressure may bemeasured during and/or closely following delivery of electricalstimulation. The detected change in pressure may be related tostimulation of the phrenic nerve.

Undesirable stimulation threshold measuring may be performed byiteratively increasing, decreasing, or in some way changing a voltage,current, duration, energy level, and/or other therapy parameter betweena series of test pulses. One or more sensors can monitor for undesirableactivation immediately after each test pulse is delivered. Using thesemethods, the point at which a parameter change causes undesirablestimulation can be identified as an undesirable stimulation threshold.

By way of example and not by way of limitation, the undesirablestimulation threshold for a particular electrode combination may bemeasured by delivering first test pulse using the initial electrodecombination. During and/or after each test pulse is delivered, sensorscan monitor for undesirable stimulation. For example, an accelerometermay monitor for movement of the diaphragm indicating that the test pulsestimulated the phrenic nerve and/or diaphragm muscle. If no phrenicnerve and/or diaphragm muscle stimulation is detected after delivery ofa test pulse, then the test pulse is increased a predetermined amountand another test pulse is delivered. This scanning process ofdelivering, monitoring, and incrementing is repeated until phrenic nerveand/or diaphragm muscle stimulation is detected. One or more of the testpulse parameters at which the first undesirable stimulation is detectedcan be considered to be the undesirable stimulation threshold.

Although methods to measure cardiac capture and phrenic stimulationthresholds are disclosed herein, it is contemplated that variousthresholds can be estimated instead of directly measured, asdemonstrated in FIG. 5.

The flowchart of FIG. 5 illustrates a process 500 for estimatingthresholds, such as a cardiac capture threshold or phrenic stimulationthreshold. The process 500 includes measuring 510 a capture threshold ofan initial electrode combination. The procedure for measuring 510 acapture threshold for the initial electrode combination can be doneaccording to any capture threshold measuring methods disclosed herein orknown in the art.

The process 500 of FIG. 5 further includes measuring 520 the impedanceof the initial electrode combination. The impedance of the initialelectrode combination may be measured with the capture thresholdmeasurement of the initial electrode combination.

Any method for measuring impedance for one or more electrodecombinations may be used. One illustrative example of techniques andcircuitry for determining the impedance of an electrode combination isdescribed in commonly owned U.S. Pat. No. 6,076,015 which isincorporated herein by reference in its entirety.

In accordance with this approach, measurement of impedance involves anelectrical stimulation source, such as an exciter. The exciter deliversan electrical excitation signal, such as a strobed sequence of currentpulses or other measurement stimuli, to the heart between theelectrodes. In response to the excitation signal provided by an exciter,a response signal, e.g., voltage response value, is sensed by impedancedetector circuitry. From the measured voltage response value and theknown current value, the impedance of the electrode combination may becalculated.

The process 500 of FIG. 5 further includes measuring 530 the impedanceof an alternate electrode combination. The measuring step 530 could berepeated for a plurality of different alternate electrode combinations.

The process 500 of FIG. 5 further includes measuring 540 an undesirableactivation threshold (e.g., phrenic stimulation threshold) of theinitial electrode combination. The procedure for measuring 540 theundesirable activation threshold of the initial electrode combinationmay be similar to the procedure for measuring 510 the capture thresholdof the initial electrode combination, and may be done concurrently withthe measuring 510 of the capture threshold of the initial electrodecombination.

The process 500 of FIG. 5 further includes estimating 550 a capturethreshold of the alternate electrode combination. Estimating 550 thecapture threshold of the alternate electrode combination can beperformed by using the capture threshold and the impedance of theinitial electrode combination and the impedance of the alternateelectrode combination.

Estimation of the capture threshold of the alternate electrodecombination in accordance with some embodiments described herein, isbased on the assumption that for a given pulse width, the capturethreshold voltage for the initial electrode combination and the capturethreshold voltage for the alternate electrode combination require anequal amount of current, energy or charge. The relationship between thecapture threshold voltage and current for each electrode combination canbe defined by Ohm's law as follows:V_(th)=I_(th)Z,  [1]

where V_(th) is the capture threshold voltage of the electrodecombination, I_(th) is the capture threshold current of the electrodecombination, and Z is the impedance of the electrode combination.

For the initial electrode combination, the relationship between thecapture threshold voltage and current may be expressed as:V_(th-in)=I_(th-in)Z_(in)  [2]

where, V_(th-in) is the capture threshold voltage of the initialelectrode combination, I_(th-in) is the capture threshold current of theinitial electrode combination, and Z_(in) is the impedance of theinitial electrode combination.

For the alternate electrode combination, the relationship between thecapture threshold voltage and current may be expressed as:V_(th-ex)=I_(th-ex)Z_(ex)  [3]

where, V_(th-ex) is the capture threshold voltage of the alternateelectrode combination, I_(th-ex) is the capture threshold current of thealternate electrode combination, and Z_(ex) is the impedance of thealternate electrode combination.

As previously stated, in some embodiments, the capture threshold currentof two electrode combinations having a common electrode is assumed to beabout equal, or, I_(th-in)=I_(th-ex).

The relationship between the alternate and initial capture thresholdvoltages may then be expressed as:

$\begin{matrix}{V_{{th} - {ex}} = {\frac{V_{{th} - {in}}}{Z_{in}}Z_{ex}}} & \lbrack 4\rbrack\end{matrix}$

By the processes outlined above V_(th-in), Z_(in), and, Z_(ex) aremeasured parameters, and the capture threshold voltage may be estimatedbased on these measured parameters.

The accuracy of an estimation calculation of a capture threshold for aparticular electrode combination may be increased if the measuredelectrode combination has the same polarity as the electrode combinationfor which the capture threshold is being estimated. Methods forparameter estimation, including capture threshold estimation, aredisclosed in U.S. patent application Ser. No. 11/505,645, filed on Aug.17, 2006, herein incorporated by reference in its entirety.

The process 500 of FIG. 5 further includes estimating 560 an undesirableactivation threshold of the alternate electrode combination. Theundesirable activation threshold can be a phrenic stimulation threshold,for example. Estimating 560 the undesirable activation threshold of thealternate electrode combination can be performed by using theundesirable activation threshold and the impedance of the initialelectrode combination and the impedance of the alternate electrodecombination. Estimating 550 the undesirable activation threshold of thealternative electrode combination can be performing using methodssimilar to estimating a capture threshold, as discussed and referencedherein.

Estimating a threshold, such as estimating a capture threshold and/or anundesirable activation threshold, instead of measuring the same, canprovide several advantages. For example, in some circumstances,measuring and estimating of some thresholds for a plurality of electrodecombinations can be done faster than measuring the threshold for eachelectrode combination of a plurality of electrode combinations, as oneor more test pulses do not need to be delivered for each electrodecombination. Additionally, a test pulse can be uncomfortable for apatient to experience, and therefore minimizing the number of testpulses can be preferable.

The methods and devices disclosed herein can employ strength-durationrelationship information measured or otherwise provided.

Capture is produced by pacing pulses having sufficient energy to producea propagating wavefront of electrical depolarization that results in acontraction of the heart tissue. Generally speaking, the energy of thepacing pulse is a product of two energy parameters—the amplitude of thepacing pulse and the duration of the pulse. Thus, the capture thresholdvoltage over a range of pulse widths may be expressed in a capturestrength-duration plot 610 as illustrated in FIG. 6.

Undesirable activation by a pacing pulse is also dependent on the pulseenergy. The phrenic stimulation strength-duration plot 620 forundesirable activation may have a different characteristic from thecapture strength-duration and may have a relationship between pacingpulse voltage and pacing pulse width.

A CRM device, such as a pacemaker, may have the capability to adjust thepacing pulse energy by modifying either or both the pulse width and thepulse amplitude to produce capture. Identical changes in pacing pulseenergy may cause different changes when applied to identical therapiesusing different electrode combinations. Determining a capturestrength-duration plot 610 can aid in characterizing the relationshipsbetween device parameter limits, capture threshold, and/or phrenicstimulation threshold, among other things.

FIG. 6 provides graphs illustrating a capture strength-duration plot 610associated and a phrenic stimulation strength-duration plot 620associated with an undesirable diaphragmic activation. A pacing pulsehaving a pulse width of W₁ requires a pulse amplitude of V_(cl) toproduce capture. A pacing pulse having pulse width W₁ and pulseamplitude V_(cl) exceeds the voltage threshold, V_(ul), for anundesirable diaphragmic activation. If the pulse width is increased toW₂, the voltage required for capture, V_(c2), is less than the voltagerequired for undesirable diaphragmic activation, V_(u2). Therefore,pacing pulses can be delivered at the pacing energy associated with W₂,V_(c2) to provide capture of the heart without causing the phrenicstimulation.

The area to the right of the intersection 651 of the capture and phrenicstimulation strength-duration plots 610, 620, between the phrenicstimulation strength-duration 620 and capture strength-duration 610plots, defines a set of energy parameter values that produce capturewhile avoiding phrenic stimulation. Pacing pulses within this regionproduce the most ideal therapy response (capture without undesirablestimulation).

The capture and phrenic stimulation strength-duration plots 610, 620 ofFIG. 6 may be generated by delivering a number of test pulses at variousamplitudes and pulse widths and evaluating whether cardiac capture andundesirable stimulation occurred. The capture and phrenic stimulationstrength-duration plots 610, 620 curves can then be completed byinterpolation and extrapolation based on, for example, an exponentialfit. Such methods can minimize the number of test pulses required tofully characterize the relationships between pulse parameters andstimulation, thereby minimizing battery consumption and uncomfortabletesting.

Extrapolation and interpolation can also allow the relationships betweenpulse parameters and stimulation for a particular device configurationto be characterized beyond what the device itself is programmed to, orcapable of, performing.

Dashed line curves 640 and 680 illustrate device capability pacingparameter limits. Maximum curve 640 illustrates the maximum energyoutput (based on pulse amplitude and width parameters) that the deviceis capable of delivering. Maximum curve 640 demonstrates a pulseparameter tradeoff for a particular device when attempting to deliverthe maximum amount of energy possible—pulse amplitude is sacrificed forgreater pulse width.

Minimum curve 680 illustrates the minimum energy output (based on pulseamplitude and width parameters) that the device is capable ofdelivering. A device can be capable of delivering pulses havingamplitudes and pulse widths parameters within the area between thecurves 640 and 680.

As demonstrated in FIG. 6, a particular device may not be capable ofdelivering a pacing pulse having any particular amplitude/widthparameters that will capture the targeted cardiac tissue without causingundesirable stimulation.

It is generally desirable to have the greatest amount of overlap betweenthe ranges of pacing pulse parameters that capture targeted cardiactissue without causing undesirable stimulation and the ranges of pacingpulse parameters that a particular device is actually capable ofdelivering, as the amount of overlap reflects the relative amount ofvariation in parameters that can be used to achieve an intended therapyoutcome.

The methods and devices discussed herein can facilitate understandingthe relationships between device parameter limits, capture threshold,and/or phrenic stimulation threshold and optimizing a therapy. Forexample, the generation of the plots of FIG. 6 can allow for acomparison of overlap between the ranges of pulse parameters thatcapture targeted cardiac tissue without causing undesirable stimulationand the ranges of pulse parameters that a particular device is actuallycapable of delivering for different device configurations. A physician(or program) may elect to use the device configuration (e.g., electrodecombination corresponding to a vector) having the greatest amount ofoverlap, as this configuration would likely correspond to theconfiguration having the greatest amount of flexibility in operation asthe possible parameter ranges that achieve a desired therapy outcome aregreatest.

Establishing the relationships between device parameter limits, capturethreshold, and/or phrenic stimulation threshold can also aid isselecting pacing parameters. For example, when selecting a pulse widthparameter, a physician may view a plot similar to that of FIG. 6 toselect the pulse width that has the greatest amplitude range, the rangebeing limited by the maximum device parameter curve 640, the minimumdevice parameter curve 680, the undesirable activation threshold curve620, and/or the capture threshold curve 610. Likewise, a pulse amplitudeparameter may be selected based on which pulse amplitude corresponds tothe greatest pulse width range within the maximum device parameter curve640, the minimum device parameter curve 680, the undesirable activationthreshold curve 620, and/or the capture threshold curve 610. Parameterselection in this way may be performed by a human or automatically by aprocessor executing program instructions.

Methods and systems for determining and using strength-durationrelationships are described in United States Patent ApplicationPublication No. 2008/0071318, filed Sep. 14, 2006, which is incorporatedherein by reference in its entirety.

FIG. 6 also illustrates programmed parameter limits 690 defining maximumand minimum pulse widths and amplitudes within which a device isprogrammed to operate. Various automated device features canautomatically change pulse parameters to adjust to various conditions,such as with an autocapture program. A doctor may implement programmedparameter limits 690 to ensure that a device does not automaticallyadjust a parameter to a level that could be detrimental to patient care,such as to a level that prematurely runs down a battery or risks causingundesirable stimulation.

Programmed parameter limits 690 may be preprogrammed or set at deviceimplantation based on detected threshold levels. If embodiments of theinvention identify changing relationships between programmed parameterlimits 690, the capture strength-duration plot 610, and the phrenicstimulation strength-duration plot 620, the programmed parameter limits690 may be adjusted, either automatically or after a doctor is notifiedof the change, for example. Adjustment of programmed parameter limits690 may increase a maximum pulse amplitude, decrease a maximum pulseamplitude, increase a minimum pulse amplitude, decrease a minimum pulseamplitude, increase a maximum pulse duration, decrease a maximum pulseduration, increase a minimum pulse duration, and/or decrease a minimumpulse duration. In such a way, a programmed parameter limit range, suchas amplitude range, may be widened, narrowed, and/or shifted within theparameter limits 640 and 680 that the device is capable of delivering byreprogramming. Other pulse parameters limits of other pulse parametersdiscussed herein or otherwise made known may be similarly reprogrammed.

In some embodiments of the invention, identification of changingrelationships between thresholds and programmed pulse limits may causethe device to modify automated processes that use pulse parameterincrements and/or scanning techniques, such as autocapture. For example,if the phrenic stimulation strength-duration plot 620 was to decreaseover time corresponding to a detected decrease in phrenic stimulationthreshold, then an autocapture program may employ smaller parameterincrements when operating within the programmed parameter limits 690.Alternatively, if the phrenic stimulation strength-duration plot 620 wasto increase over time corresponding to a detected increase in phrenicstimulation threshold, then autocapture parameter increments may beincreased. Increases in increments can facilitate faster identificationof thresholds and the like while minimizing the delivery of test pulses.Decreases in increments can allow for more cautious and thoroughscanning. Increasing or decreasing pulse increments in response tochanges in relationships between programmed parameter limits andthresholds can quickly optimize automated device functions whilebalancing safety, efficacy, and battery consumption considerations.Changes in parameter increments may be made automatically by a deviceupon detection of a change in relationship between programmed parameterlimits and thresholds and/or implemented by a doctor upon reviewinginformation regarding the identified change in relationship.

In some embodiments, a capture threshold and/or phrenic stimulationthreshold may be periodically identified and updated. If some amount ofparameter separation exists between the programmed parameter limits andone or both of the thresholds then a device may retest to identify thecapture threshold and/or phrenic stimulation threshold less frequently.The separation can be a preprogrammed safety margin between thresholdsand programmed parameter limits. If a detected threshold is identifiedas within the parameter separation (e.g., exceeding the safety margin)then a device may increase the frequency with which it tests thethresholds. Increasing and/or decreasing the frequency of testing basedon proximity of detected thresholds to programmed parameter limits canminimize battery consumption and uncomfortable testing while balancingsafety and efficacy considerations (i.e., testing is done morefrequently when it is likely that a threshold will drift into theprogrammed parameter limits and less frequently when a large marginexists between a threshold and the programmed limits).

The flowchart of FIG. 7 illustrates a process 700 for using phrenicstimulation algorithms for identifying, and characterizing therelationships between, device parameter limits, capture threshold,and/or phrenic stimulation threshold, among other things. The process700 includes initiating 710 a capture/threshold test and setting aninitial pacing parameter. The initial pacing parameter setting can be,for example, a device minimum amplitude, a device minimum pulse width, adevice minimum pulse current, a previously determined capture threshold,or some combination thereof. The process 700 further includes delivering720 at least one pacing pulse using the current pacing parametersetting. The current pacing parameter setting can be the initial pacingparameter setting if step 720 is being performed for the first time.Otherwise, the current pacing parameter setting can be a parameter value(e.g., pulse amplitude) different from that of the initial settingvalue.

After delivery 720 of the at least one pacing pulse, a phrenic sensorsignal can be obtained 730. Such a phrenic sensor signal can be anysignal produced by any sensor that is capable of detecting phrenicstimulation. The phrenic sensor signal is then evaluated 740. Theevaluation 740 can be used to determine whether a delivered 720 pacingpulse stimulated the phrenic nerve or otherwise cause diaphragmicmovement. If phrenic stimulation is detected 750, then phrenicstimulation threshold/device information is stored 760. Such informationcan reflect that the phrenic stimulation threshold (PST) is less than adevice maximum parameter value, and may be equal to the device parameterminimum value or capture threshold, if the parameter value had beenaccordingly set and increased.

If phrenic stimulation is not detected 750, then it is determinedwhether the pacing parameter setting is set at a maximum value 770.During the first few iterations of the process 700, it is unlikely thatthe pacing parameter setting is set at a maximum value 770, and in whichcase the process 700 increments 790 the current pacing pulse parameterand returns to delivering 720 at least one more pacing pulse using thecurrent pacing parameter setting. In this way, the process 700 canrepeat steps 720-730-740-750-770-790, increasing the pacing pulseparameter each iteration in a scanning fashion until a phrenicstimulation threshold is identified 750-760 or the pacing parametersetting reaches a maximum 770.

If the pacing parameter setting is incremented 790 to a maximum 770,then the process 700 stores 780 phrenic stimulation threshold/deviceinformation. Such phrenic stimulation threshold/device information couldreflect that the PST is greater than the maximum device parametersetting.

The capture threshold and/or PST for a particular electrode combinationmay change over time due to various physiological effects. Testing thecapture threshold and PST for a particular electrode combination may beimplemented periodically or on command to ensure that the informationregarding relationships between device parameter limits, capturethreshold, and/or phrenic stimulation threshold is current.

The flowchart of FIG. 8 illustrates a process 800 for using phrenicstimulation algorithms for identifying, and characterizing therelationships between, device parameter limits, capture threshold,and/or phrenic stimulation threshold, among other things. The process800 includes initiating 810 a capture/threshold test and setting aninitial pacing parameter. The initial pacing parameter setting can be,for example, a device maximum amplitude, a device maximum pulse width, adevice maximum pulse current, a previously determined threshold, or somecombination thereof. The process 800 further includes delivering 820 atleast one pacing pulse using the current pacing parameter setting. Thecurrent pacing parameter setting can be the initial pacing parametersetting if step 820 is being performed for the first time. Otherwise,the current pacing parameter setting can be a parameter value (e.g.,pulse amplitude) different from that of the initial setting value.

After delivery 820 of the at least one pacing pulse, a phrenic sensorsignal can be obtained 830. Such a phrenic sensor signal can be anysignal produced by any sensor that is capable of detecting phrenicstimulation. The phrenic sensor signal is then evaluated 840. Theevaluation 840 can be used to determine whether a delivered 820 pacingpulse stimulated the phrenic nerve. If phrenic stimulation is detected850 then phrenic stimulation threshold/device information is stored 860.Such information can reflect that the PST is greater than or equal to adevice maximum parameter value.

If phrenic stimulation is not detected 850, then it is determinedwhether the pacing parameter setting is set at a minimum value and/orcapture threshold 870. During the first few iterations of the process800, it is unlikely that the pacing parameter setting is set at aminimum value or capture threshold 870, and in which case the process800 decrements 890 the current pacing pulse parameter and returns todelivering 820 at least one more pacing pulse using the current pacingparameter setting. In this way, the process 800 can repeat steps820-830-840-850-870-890, decreasing the pacing pulse parameter eachiteration in a scanning fashion until a phrenic stimulation threshold isidentified 850-860 or the pacing parameter setting reaches a deviceminimum and/or capture threshold 870. In some embodiments, it may bedesirable to not scan for the phrenic stimulation threshold below thecapture threshold as it could be dangerous to lose capture of cardiactissue during the test.

If the pacing parameter setting is decremented 890 to a device minimumand/or capture threshold 870, then the process 800 stores 880 phrenicstimulation threshold/device information. Such phrenic stimulationthreshold/device information could reflect that the PST is less than theminimum device parameter setting and/or cardiac capture threshold. Ifthe testing fails to identify pacing parameters that produce capture andavoids phrenic stimulation, then an alert may be communicated to theexternal device via communication circuitry to alert a system orphysician.

The flowchart of FIG. 9 illustrates a process 900 for using phrenicstimulation algorithms for identifying, and characterizing therelationships between, device parameter limits, capture threshold,and/or phrenic stimulation threshold, among other things. The process900 includes initiating 910 a capture/threshold test, and setting aninitial pacing parameter. The initial pacing parameter setting can be,for example, a device maximum amplitude, a device maximum pulse width, adevice maximum pulse current, a previously determined threshold, or somecombination thereof. The process 900 further includes delivering 920 atleast one pacing pulse using the current pacing parameter setting. Thecurrent pacing parameter setting can be the initial pacing parametersetting if step 920 is being performed for the first time. Otherwise,the current pacing parameter setting can be a parameter value (e.g.,pulse amplitude) different from that of the initial setting value.

After delivery 920 of the at least one pacing pulse, a phrenic sensorsignal and cardiac sensor signal can be obtained 930. Such phrenic andcardiac sensor signals can be any of the signals produced by sensorsthat are capable of detecting phrenic stimulation or detecting cardiaccapture. The phrenic sensor signal and cardiac sensor signal are thenevaluated 940. The evaluation 940 can be used to determine 945 whether adelivered 920 pacing pulse stimulated the phrenic nerve. If phrenicstimulation is detected 945 then the process 900 determines whethercapture was lost 946 during the delivery 920 of the one or more pacingpulses.

If phrenic stimulation 945 and loss of cardiac capture 946 are bothdetected then cardiac capture and phrenic stimulation threshold/deviceinformation can be stored 970. Such information can reflect that thephrenic stimulation threshold PST is less than the capture threshold.

If phrenic stimulation is detected 945 and loss of capture is notdetected 946 then the current pacing pulse parameter is decremented 980.For example, if the pacing pulse parameter is pulse width, then thecurrent pulse width can be decremented 980 to a shorter pulse width.Pulse amplitude, frequency, and/or current pulse parameters can besimilarly decremented (or incremented, in step-up embodiments).

If phrenic stimulation is not detected 945, the process 900 determineswhether loss of capture was detected 950. If no phrenic stimulation isdetected 945 and loss of capture is detected 950, then capture andphrenic stimulation threshold/device information is stored 960. Suchinformation can indicate that the phrenic stimulation threshold isgreater than or equal to the capture threshold.

In this way, the process 900 can repeat steps 920-930-940-945-946-980,or steps 920-930-940-945-950, decreasing the pacing pulse parameter eachiteration in a scanning fashion until a relationship between phrenicstimulation threshold is the capture threshold is identified (e.g.,PST>capture threshold or PST<capture threshold). Such a process allowsfor the simultaneous scanning for both a phrenic stimulation thresholdand the cardiac capture threshold. Searching for these parameterstogether minimizes the number of pulses that need to be delivered, ascompared to doing the tests separately, preserving battery energy andminimizing patient discomfort. Simultaneous scanning in this way alsominimizes the total time necessary for a device to test to establishthese thresholds.

The processes 700, 800, and 900, as well as other methods discussedherein, can be initiated upon implant, by a physician, upon detection ofa change in condition, and/or periodically. Condition changes that couldinitiate the processes include loss of capture, change in posture,change in disease state, detection of non-therapeutic activation, and/orshort or long term change in patient activity state, for example.

The device parameter limits of FIGS. 7, 8, and 9 as well as othersdiscussed herein, can be programmed parameter limits or parameter limitscorresponding to maximum/minimum pulse parameter values that a pacingsystem is capable of delivering.

Periodic and/or condition initiated testing to update capture threshold,phrenic stimulation threshold, and device relationship information canbe useful to monitor for certain conditions that might not otherwise bereadily apparent but warrant attention and/or a therapy change. Deviceand/or physiologic changes may alter the effect of pacing pulses. Forexample, device component defects, lead migration, electrodeencapsulation, and/or physiologic changes may increase the pacing pulseamplitude needed to reliably produce capture and/or decrease the pacingpulse amplitude needed to stimulate the phrenic nerve, leading touncomfortable and ineffective pacing therapy. Updated capture threshold,phrenic stimulation threshold, and device relationship information canbe used to automatically reprogram the therapy device and/or alert aphysician to reconfigure the therapy device.

The various processes illustrated and/or described herein (e.g., theprocesses of FIGS. 1,5,7, 8, and 9 and those associated with FIG. 6) canbe performed using a single device embodiment (e.g., device of FIGS. 2and 4) configured to perform each of the processes discussed herein.

The components, functionality, and structural configurations depictedherein are intended to provide an understanding of various features andcombination of features that may be incorporated in an implantablepacemaker/defibrillator. It is understood that a wide variety of cardiacmonitoring and/or stimulation device configurations are contemplated,ranging from relatively sophisticated to relatively simple designs. Assuch, particular cardiac device configurations may include particularfeatures as described herein, while other such device configurations mayexclude particular features described herein.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

What is claimed is:
 1. A method of controlling operation of a medicaldevice, the method comprising: delivering a pulse with a predeterminedpulse energy as part of a cardiac capture threshold test; determining ifthe delivered pulse produced phrenic stimulation; continuing the capturethreshold test when it is determined that the delivered pulse did notproduce phrenic stimulation; and halting the capture threshold test whendetecting, using the medical device, that the delivered pulse didproduce phrenic stimulation.
 2. The method of claim 1, further includingstoring phrenic stimulation information in the medical device whendetecting that the delivered pulse did produce phrenic stimulation. 3.The method of claim 2, wherein the storing the phrenic stimulationinformation includes storing a comparison of a phrenic stimulationthreshold to one or both of a maximum device pulse amplitude and aminimum device pulse amplitude.
 4. The method of claim 2, wherein thestoring the phrenic stimulation information includes storing acomparison of a phrenic stimulation threshold to a capture thresholdused in the capture threshold test.
 5. The method of claim 1, whereinthe predetermined pulse energy corresponds to a cardiac pacing devicecapability limit.
 6. The method of claim 1, wherein the predeterminedpulse energy corresponds to a programmed cardiac pacing device limit. 7.The method of claim 1, wherein a cardiac pacing device includes aplurality of electrode combinations where each electrode combinationdefines a pacing vector, the method comprising performing the method foreach of two or more pacing vectors.
 8. The method of claim 7, furthercomprising: for each of the two or more pacing vectors, displaying on adisplay of a patient external device an indication of whether the pulseproduced phrenic stimulation; and for each of the two or more pacingvectors, displaying on the display of the patient external device anindication of a capture threshold as determined by the capture thresholdtest.
 9. A medical apparatus, comprising; a pulse generator forgenerating pulses at selectable pulse energy; and a controlleroperatively coupled to the pulse generator, the controller configuredto: initiate delivery of a pace pulse with a predetermined pacing pulseenergy as part of a cardiac capture threshold test; determine when thedelivered pulse produced phrenic stimulation; continue the capturethreshold test when it is determined that the delivered pulse did notproduce phrenic stimulation; and halt the capture threshold test whendetermining that the delivered pulse did produce phrenic stimulation.10. The medical apparatus of claim 9, including a memory circuitintegral to, or operatively coupled to the controller, wherein thecontroller is configured to store phrenic stimulation information in thememory circuit when detecting that the delivered pulse did producephrenic stimulation.
 11. The medical apparatus of claim 10, wherein thephrenic stimulation information includes a comparison of a phrenicstimulation threshold to one or both of a maximum device pulse amplitudeand a minimum device pulse amplitude.
 12. The medical apparatus of claim10, wherein the phrenic stimulation information includes a comparison ofa phrenic stimulation threshold to a capture threshold used in thecapture threshold test.
 13. The medical apparatus of claim 9, whereinthe predetermined pulse energy corresponds to a cardiac pacing devicecapability limit.
 14. The medical apparatus of claim 9, wherein thepredetermined pulse energy corresponds to a programmed cardiac pacingdevice limit.
 15. The medical apparatus of claim 9, including aplurality of electrodes configurable as a plurality of electrodecombinations, wherein an electrode combination defines a pacing vector,and wherein the controller is configured to deliver a pace pulse to twoor more pacing vectors and determining whether the delivered pulseproduced phrenic stimulation.
 16. The medical apparatus of claim 15,further comprising: for the two or more pacing vectors, displaying on adisplay of a patient external device an indication of whether thedelivered pulse produced phrenic stimulation, and an indication of acapture threshold as determined by the capture threshold test.
 17. Amethod, comprising; (a) selecting an electrode combination of aplurality of electrode combinations; (b) delivering a pulse with apredetermined pacing pulse energy via the selected electrodecombination; (c) determining if the delivered pulse produced phrenicstimulation; (d) continuing the capture threshold test when it isdetermined that the delivered pulse did not produce phrenic stimulation;(e) halting the capture threshold test when detecting, using the medicaldevice, that the delivered pulse did produce phrenic stimulation; and(f) repeating steps (a)-(e) for at least one other electrode combinationof the plurality of electrode combinations.
 18. The method of claim 17,further comprising: displaying a capture threshold as determined by thecapture threshold test for the selected electrode combination on adisplay of a patient external device.
 19. The method of claim 17,further comprising: displaying a phrenic stimulation threshold for theselected electrode combination on a display of a patient external devicewhen determining that the delivered pulse did produce phrenicstimulation.
 20. The medical apparatus of claim 17, further comprising:a switch for selectively connecting the pulse generator to a selectedelectrode combination of a plurality of electrode combinations; andwherein the controller is configured to repeat (a)-(d) for at least oneother electrode combination of the plurality of electrode combinations.